1. Field of the Invention
The present invention related to a differential detection receiver used in a digital radio communication and, more specifically, to simplification of constituent circuits and the entirety of such a receiver.
2. Description of the Prior Art
In order to effectuate differential detection with a high precision in a digital circuit, it is a common practice to multiply a symbol in a signal with a just preceding symbol in the signal by using a multiplier and to subsequently perform an addition or subtraction operation to the obtained product. However, a multiplier requires a large scale circuit and accordingly a large amount of electric power, which is especially true if differential detection is to be achieved at a high speed. For this reason, in order to effectuate differential detection without using a multiplier, there is adopted a scheme in which differential information is detected by finding an arctangent (tan.sup.-1) of each symbol in a signal from a conversion table and calculating the difference between arctangents of adjacent symbols. Since this scheme fails to reduce the circuit size if the scheme requires a large conversion table, various techniques have been devised which eliminate the need of storing a lot of data in a conversion table.
Japanese Patent No. Sho62-529 (1987) discloses a digital arithmetic circuit which, for a given vector, calculates the magnitude thereof and the angle thereof with a reference coordinate as an inverse trigonometric function or an arctangent by using a reduced-size arctangent conversion table.
Japanese Patent No. Hei6-105,421 (1994) discloses a digital circuit for calculating an inverse trigonometric function of an arctangent of a 2n-bit binary number X. In this system, the calculation is achieved by utilizing the fact that if the binary number X comprises n higher digits H and n lower digits L (X=H+L), then the arctangent of X can be approximated as EQU arctan (X)=arctan (H)+L/(H.sup.2 +1).
This system requires two conversion tables for arctan (H) and 1/(H.sup.2 +1) and a multiplier.
Further, in demodulating an input signal, the input signal needs level adjustment. In order to adjust the level of the input signal, the gain of an input signal amplifier is generally controlled by feeding back the result of comparison between a coded output and a reference level to the input signal amplifier.
Japanese Patent unexamined publication No. Heil-71,270 (1989) discloses a level adjusting device of a just mentioned type. The disclosed device comprises a differential amplifier, an A/D converter, an LPF, a gate, a gate pulse generator, a subtracter, a nonlinear amplifier, an integrator and a D/A converter.
However, the above mentioned arctangent calculating circuits still require conversion tables and multipliers and the last mentioned device requires a D/A converter, which prevents the reduction in size and power consumption of the circuit. Thus, there remain in the prior art some room for improvement to reduce the size and the power consumption of constituent circuits of a differential detection receiver by further simplifying of a differential detection receiver by further simplifying such circuits.